A gas is heated during its adiabatic compression. Due to the heating and the associated increase in volume, the requisite compression work is increased. This has various adverse consequences at the compressor of a gas turboset. The increased inlet temperature at the combustor can result in lower firing of the gas turboset. The compression requires a greater proportion of the turbine work and takes place at a poorer efficiency. In addition, cooling air branched off from the compressor is available only at an already greatly increased temperature level. Consequently, the output and efficiency potentials of the gas turboset are adversely affected.
Attempts are consequently made to limit this temperature increase, for example by intermediate-cooling stages during the compression. The “isothermal compression” is also of interest in this connection. The injection of a liquid, in particular water, into the compressor or into its inflow (the latter injection being especially simple to realize) in such a way that liquid droplets enter the compressor is in this case an especially simple means of achieving internal cooling of the compressor by the evaporation of this liquid. By a liquid being sprayed into the compressor or into the regions located upstream of the compressor, a similar cooling effect as with an intermediate-cooler heat exchanger can be achieved by the evaporation inside the compressor. This involves an approximation to the “quasi-isothermal compression.” On account of the lower temperatures achieved by means of the quasi-isothermal compression and the associated higher specific density of the gas to be compressed, less energy is required for the compression. FR 1563749 has already described the positive effects of the injection of liquid into compressors, but at the same time emphasizes the importance of fine atomization and uniform distribution of the liquid.
It is not actually unusual for water to penetrate into a compressor during operation, thus during the cleaning of a stationary gas turbine or in an aircraft engine when flying through clouds or rain. However, since permanent water injection could lead to problems on account of blade erosion, the sprayed liquid must be atomized very finely into very small droplets. Therefore the challenge with corresponding technical solutions has hitherto been to realize water spraying with very small droplets, normally 0.9-5 μm in diameter during “flash atomization” and 20-40 μm during high-pressure atomization. At the same time, the sprayed quantity of these droplets must be so large that it is sufficient for cooling the air during the compression.
WO 99/67519 discloses the “swirl-flash technology” for generating very fine droplets. It is based on the fundamental principle that a liquid is pressurized, superheated and then sprayed by means of a nozzle. The liquid, in particular water, discharges from the nozzle in a typical cone. The droplet size is approximately 25 μm. Since the temperature of the liquid is considerably above the boiling point at ambient pressure, spontaneous boiling occurs in such a way that each droplet, during the spraying process, explodes from 25 μm into about 1000 fragments having a size of approximately 2.5 μm.
For the intended purpose, the kinetics of the evaporation are also of importance. The retention time in a compressor is short. It is normally around 10 milliseconds for an axial-flow compressor, and is even lower for a radial-flow compressor. This means that the evaporation has to be effected within milliseconds. As already explained, the very fine droplets in combination with the high temperature permit the desired rapid evaporation.
A device and a method for spraying a cooling medium into the supply air flow of a gas turbine plant are described in U.S. Pat. No. 5,930,990. In this case, the injection nozzles are fastened in a lattice shape on or in a tube carrier arrangement. This tube carrier arrangement, via which the nozzles are also supplied with liquid, may be arranged at various points in the inflow duct of the gas turboset. The nozzles may in this case be arranged as a function of the respective flow parameters. In addition, it is possible in each case to combine a plurality of nozzles to form a group, so that the injected liquid quantity can be adapted, for example, to the variable fresh-air flow by multistage switching-on of the nozzle groups. The variation in the liquid mass flow by multistage switching-on has the advantage that the pressure drop over the active nozzles and thus the atomizing quality remain largely constant during different liquid mass flows. On the other hand, the profile of the droplet load of the intake air may greatly vary and have steep gradients due to the switching-on and switching-off of various nozzle groups.
In addition to a suitable design of the nozzles, it is necessary to spray liquid droplets into the inflow as homogeneously as possible over the entire inflow cross section. In addition to the atomizing quality, FR 1563749 also mentions that the homogeneous, uniform distribution of the liquid introduced is also decisive. A distribution which is very uneven results in locally varying cooling in the compressor. The resulting warm and cold strands reduce the pumping distance (surge margin) of the compressor and in the extreme case may lead to the distortion of the casing.